As is known, Raschel-type linear knitting machines with double needle-bed are provided with two rows of needles, each one being housed in a respective needle-bed and supported by a respective bar. Every needle-bed is further equipped with a series of latches that can be coupled to the needles for so-called “compound” systems, and a series of “stitch-combs” commonly used for preventing the loop from being taken upwards when the needle gets up for discharging the stitch and taking a new thread. Also the series of latches and stitch-combs are supported by respective bars, which are parallel to the needle bars and develop longitudinally along the whole machine.
Needle bars, latch bars and stitch-comb bars are moved synchronously with a forward-backward motion by means of (more or less complex) systems consisting of compound levers suitably arranged (in the form of quadrilaterals), accurately calculated and apt to make the movements they are designed for.
In particular, examples of known machines belonging to this type can be found in documents U.S. Pat. No. 3,221,520, U.S. Pat. No. 3,568,470, U.S. Pat. No. 3,460,358, U.S. Pat. No. 4,332,149, U.S. Pat. No. 3,950,942, DE 3620259 and WO03/071018.
Modern computer-assisted design technologies enables to study accurately the laws of motion of said compound levers with a perfect synchronism between the various fabric-forming elements, starting from the same cam set into motion by a drive shaft.
This system is based on very delicate balances between levers, fulcrums, speeds, accelerations, paths, trajectories and any other factor constituting the mechanism as a whole.
Moreover, said compound levers are designed by analyzing the shape changes of the concerned elements due to working defects, temperature changes and clearances that cannot be avoided in order to operate the knitting machine within the tolerance limits the knitting system can tolerate. As a matter of fact, it should be pointed out that yarns have given limits of mechanical resistance, within which they have to be worked so as to avoid their breaking, stress deformation, bad fabric-formation, which all mean bad quality. Therefore, compound levers are also designed as a function of the type of thread used and of the thread tensioning and feeding values determining the fabric width.
That is why the machine is normally carried out for manufacturing a given item according to the customer's needs. In other words, movements and size of the compound levers are adjusted and calibrated (in textile jargon “sampled”) for manufacturing a specific item. However, known machines as described above have great drawbacks.
A first drawback is the little versatility of said machines for manufacturing different items.
As a matter of fact, it should be pointed out that in order to change the layout of the machine so as to obtain movements differing from the predefined ones, very delicate and complex adjustments are carried out, requiring the intervention of personnel having specific instruments and knowledge. For instance, if a yarn whose characteristics differ from the ones previously worked is used, the compound levers should be modified by specialized personnel, changing the path of the elements involved in fabric formation.
As a matter of fact, needles, latches and stitch-combs (which are regarded as consumables) can be replaced by personnel working in the knitwear factory; whereas for other parts, such as for instance a cam replacement, personnel working in the manufacturing firm should intervene, since this involves various disassembling and recalibrating operations.
That is why only a technician working for the firm manufacturing the knitting machine can make changes, if necessary, to said knitting machine trying to simulate and calculate possible implications of the system as a function of the changes required by the customer.
The intervention of specialized personnel is highly expensive because of idle times in which the machine is not used and of costs involving operators' transfer.
A further disadvantage of knitting machines as described above consists in that the mechanisms of movement should necessarily be carried out with highly precise physical elements that show almost inexistent deformations under stress. This means a high accuracy in the choice of materials, shapes, size, working tolerances, which results in high manufacturing costs.
Moreover, it should also be pointed out that the aforesaid compound levers, beyond being quite bulky, require quite large spaces for their movement. Also the positioning of the cams, as is known spaced from one another, and the positioning of the motor involve a particularly large overall size of the whole machine.
A further disadvantage consists in the shape of the rotating shaft associated to the control cams and set into rotation by the motor. Said shaft, which extends on the whole longitudinal development of the machine (needlebeds have a length above 3.5 meters), involves serious problems as far as working, balancing, vibration reduction, assembly are concerned, which problems result exactly from the size of said shaft.